When hydrocarbons are produced from wells that penetrate hydrocarbon producing formations or zones, water often accompanies the hydrocarbons, particularly as the wells mature in time. The water can be the result of a water producing zone communicating with the hydrocarbon producing formations or zones by fractures, high permeability streaks and the like, or the water can be caused by a variety of other occurrences which are well known to those skilled in the art such as water coning, water cresting, bottom water, channeling at the well bore, etc. It becomes an economic necessity to improve the hydrocarbon/water ratio during hydrocarbon production such that recovery remains cost effective.
In enhanced recovery techniques such as water flooding, an aqueous flood or displacement fluid is injected under pressure into oil containing subterranean formations or zones by way of one or more injection wells. The flow of the aqueous fluid through the formations or zones displaces hydrocarbons contained therein and drives them to one or more producing wells. However, the aqueous displacement fluid often flows through the most permeable formations or zones whereby less permeable formations or zones containing hydrocarbons are bypassed. This uneven flow of the aqueous displacement fluid through the formations or zones reduces the overall yield of hydrocarbons therefrom. Heretofore. enhanced recovery problems in subterranean hydrocarbon containing formations or zones caused by permeability variations therein have been corrected by reducing the permeabilities of the subterranean flow paths having high permeabilities and low hydrocarbon content. As a result, the subsequently injected aqueous displacement fluid is forced through flow paths having low permeability and high hydrocarbon content.
There has been a continuing and long-felt need for improving the oil/water ratio during hydrocarbon production by using chemical gel systems to resist the flow of injected or natural aqueous drive fluid through high permeability channels and fractures. Techniques utilized to improve the hydrocarbon/water ratio and/or to reduce high flow path permeability during enhanced recovery operates are referred to in the art as “conformance control techniques.”Conformance control techniques have also been used to modify the gas permeability of a formation. The general approach of most conformance control techniques has been to inject a mixture of reagents, initially low in viscosity, into a zone of the formation that has high permeability. After a sufficient time to allow the mixture to be pumped into the subterranean formation or when exposed to the elevated temperature of the formation, the mixture of reagents then forms a barrier to at least partially block the flow of water and/or gas through the zone. As used herein, the term “zone” simply refers to a portion of a formation and does not imply a particular geological strata or composition. For example, previous methods have included injecting aqueous solutions of polymers and activators into the high permeability flow paths whereby the polymers are gelled and cross-linked therein. Water soluble polymers including copolymers of acrylamide and acrylic acid cross-linked with transition metal ions are among the systems that have been used.
One drawback to some of the gels that have previously been used as conformance control materials or as other types of subterranean treatments, e.g., sand consolidation treatments, is that those gels may be viewed as unsuitable for subterranean use in some areas according to certain environmental protection guidelines and regulations. Another drawback is that treatment fluids that comprise conventional polymers and activators may not gel sufficiently at the low temperatures that may be encountered in some subterranean formations, e.g., temperatures below about 20° C. One substance that may meet even rigorous environmental guidelines for use in subterranean operations is colloidal silica.
Heretofore, colloidal silica has been used as an additive in cements that are intended for subterranean cementing operations. For example, colloidal silica has been added to subterranean cements as an extender and viscositier. Colloidal silica has also been added to subterranean cement slurries to prevent the migration of gas through the cement slurry in the fluid-to-solid transition phase. One such commercially available cement additive that comprises colloidal silica is the product commercially available from Halliburton under the trade name GASCON 469™. Another such commercially available cement additive that comprises colloidal silica is the product commercially available from Halliburton under the trade name FDP-C725-04. Most of the prior art cements have been substantially free of substances that would cause the colloidal silica in a cement slurry to accelerate the formation of a colloidal silica gel. Hence, the cement has hardened before such a reaction has taken place.
It is believed that WO 2004/0138381, at least in some respect, teaches an injection grouting technique in which a composition comprising colloidal silica particles, alkali metal silicate or organic silicate, and at least one gelling agent that is capable of gelling colloidal silica is injected into distinct permeable areas referred to therein as “cavities or leaking parts such as . . . fissures [or] cracks.” Id. at page 2, lines 33-35. One proposed use for the injection grouting technique is to seal openings in “constructions such as . . . well cementing.” Id. at page 2, lines 33-37. It is apparent from the disclosure of WO 2004/0138381 that the injection grouting of well cement would involve sealing a hole that has developed in a pre-existing well cement construction, e.g., a fracture in a pre-existing sheath of cement located in a well bore. As such, WO 2004/0138381 does not disclose the creation of a new cement sheath or the treatment of an entire well bore to effect an overall reduction in permeability of the most permeable areas of the well bore.
It is believed that WO 03/033618, at least in some respect, teaches a composition that comprises a silica sol with an S-value higher than about 72% and at least one gelling agent, and methods of using that composition by inserting it into a leaking part or cavity. According to WO 03/033618, the principle application for the composition disclosed therein is the creation of subsurface barriers to water flow. Id. at page 6, lines 8-14. For example, the application discloses inserting the composition into a microcrack or other cavity in a rock. Id. The application also discusses prior art compositions that have been used for sealing water leaks in concrete walls and tunnels, and sealing cavities that form behind concrete walls. Id. at page 1, lines 10-12, 26-29. Although WO 03/033618 indicates that the composition may be suitable for use in subsurface areas and may be able to withstand high water pressure, it does not suggest using the composition in a well bore that penetrates a subterranean formation, e.g., a well bore that penetrates a hydrocarbon-bearing formation, with the intent of producing hydrocarbons through the formation matrix (e.g., pore space).